/Engine/Source/Runtime/Slate/Public/Framework/Application/SlateApplication.h
Is the window in the app's destroy queue? If so it will be destroyed next tick.
true if Window is in the destroy queue
bool IsWindowInDestroyQueue ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWindow](API\Runtime\SlateCore\Widgets\SWindow) > Window ) const